1. Field of the Invention
The present invention relates to a sheet thickness detection device for detecting a thickness of a sheet, and relates to an image forming apparatus mounted with the sheet thickness detection device.
2. Description of the Related Art
In recent years, an investigation has been made to develop a sheet thickness detection apparatus able to accurately measure thicknesses of sheets from thin sheets such as 52 gram paper to thick sheets such as 400 gram paper. The term “52 gram paper” refers to a sheet having a basis weight of 52 grams per square meter and the term “400 gram paper” refers to a sheet having a basis weight of 400 grams per square meter.
An image forming apparatus, such as a copying machine and a printer, is mounted with a sheet conveyance roller. When a sheet passes through the conveyance roller, a rotation shaft (roller shaft) of the conveyance roller is displaced by an amount corresponding to a sheet thickness. A sheet thickness detection device has been proposed that measures a sheet thickness by detecting a displacement of the roller shaft by using a magnetic sensor, which is disposed to face a magnet attached to one end of the roller shaft (see, Japanese Laid-open Patent Publication No. 2008-254855).
Another sheet thickness detection device is disclosed in Japanese Patent Publication No. 2872022. The disclosed device has a reference roller disposed alongside a conveyance path and a detection roller disposed to face the reference roller. The detection roller is configured to be displaced to follow the thickness of a sheet passing through between the reference roller and the detection roller. The sheet thickness is detected through gears that are driven to follow the displacement of the detection roller. Further, the amount of displacement of the detection roller caused by passage of a sheet between the rollers is amplified to improve the accuracy of sheet thickness detection and to enable detection of the number of sheets being fed in multiple, if multiple feeding occurs.
However, when an attempt is made to detect the thickness of an ultra-thin sheet such as 38 gram paper by using the sheet thickness detection device disclosed in Japanese Laid-open Patent Publication No. 2008-254855, the resultant output of the magnetic sensor representing the sheet thickness becomes small.
Assuming that voltage levels of sensor output in a non sheet passage state and in a sheet passage state are respectively represented by v0 and v1, a difference value |v1−v0| represents a sensor output corresponding to sheet thickness. If the sensor output |v1−v0| is small, it is difficult to accurately detect the sheet thickness.
The sheet thickness detection device disclosed in Japanese Patent Publication No. 2872022 amplifies the amount of displacement of the detection roller caused by sheet passage at the same degree of amplification for sheets from thin sheets to thick sheets. If the linearity of sensor output characteristic is deteriorated (saturated) with increasing sheet thickness, the sensor output corresponding to sheet thickness cannot be obtained for thick sheets by the sheet thickness detection performed at the same degree of amplification irrespective of sheet thickness, so that the sheet thickness cannot accurately be detected.